This invention relates to a method of reproducing recorded images and, more particularly, to a method of carrying out special reproduction, such as reverse reproduction, of image data.
First, a television signal will be described as an example of applying the reproducing method of the present invention. Much progress in the development of still-picture broadcasting utilizing high-definition television has been made in recent years. In such still-picture broadcasting, a high-definition signal possesses five times as much image information as in current NTSC broadcast television systems, and the screen is much wider. This is advantageous in that it is possible to enjoy realistic video having greater resolution.
Even though the images broadcast are still pictures, they contain a very large amount of information. Accordingly, in consideration of the limitations possessed by broadcast electric waves and the limit on channel capacity, still-picture broadcasting has been adapted in such a manner that a still picture can be transmitted even in a narrow transmission band (or at a low transmission rate) by compressing the information using compressive coding. More specifically, unlike moving pictures, the scenes in a still picture are stationary. In addition, since the time which a viewer has to see one scene ranges from several seconds to ten-odd seconds, it has been contemplated to transmit the image data at a bit rate of 2 Mbps.
FIGS. 1A and 1B illustrate the configuration of still-frame data when the data are transmitted. When the still-frame data are transmitted, one frame each of the transmitted data comprises frame-control data, image data, and sound or voice data, as illustrated in FIG. 1A. One item of frame-control data is annexed to one item of image data (sound data).
The frame-control data in FIG. 1A refer to data which describe the method of controlling the picture. Ordinarily, the frame-control data describe the method of controlling the frame when a changeover (switching) is made from one frame to the next. In still-picture broadcasting, frame control can include simple cut-switching control, wipe control, dissolve or scroll control, in which a gradual switching is made from one frame to the next, and partial rewrite control. The frame-control data include data indicating the type of frame control, data indicating the frame display mode, and address information indicative of a position at which rewriting is performed at the time of a partial rewrite. By using this frame-control data, the frame display method can be widely diversified to provide a more enjoyable picture.
In a case where frame control such as wipe, dissolve or scrolling is being carried out, the image data (FIG. 1A) to be transmitted will include all the image data of one frame. In order to simplify the description, the combination of frame-control data and image data, in which the image data include all the image data of one frame, shall be referred to as "basic-frame data" hereinafter.
The partial rewrite mode mentioned above is a mode of the following kind: In a transition from one frame to the next, there are cases where the image data of the next frame are obtained by rewriting only a portion of the image data of the frame transmitted previously. In such cases, all of the image data of the next frame are not transmitted; rather, the image data solely of the rewritten portion are transmitted to perform the rewriting of the frame. In this partial-rewrite mode, the image-data portion includes image data solely of the portion related to rewriting. For the sake of description, the combination of frame-control data, which are used in the partial rewrite, and the partial-frame data, which include the image to be altered, shall be referred to as "collateral information data" (See FIG. 1B), as opposed to the basic-frame data mentioned above.
On the other hand, large-capacity recording media such as optical disks and opto-magnetic disks have recently become generally available. One method of utilization contemplated is to receive a-still-picture broadcast of the kind described above, record the received still picture on a recording medium of the above-mentioned kind, and reproduce or playback the still picture whenever necessary.
Accordingly, in a case where a series of transmitted images are recorded on a recording device of this type, the transmission data are recorded in the order in which they were received. In case of the frame control mentioned above, such as wipe control, dissolve control or scrolling control, and especially partial-rewrite control, the transmission sequence (or in other words, the recording sequence) is of very great importance. For example, in a case where a basic frame A and a basic frame B are dissolved, the basic-frame data of the basic frame A are transmitted first, and the basic-frame data for the basic frame B are transmitted next. The above-mentioned recording sequence is of upmost importance when the partial-rewrite mode is in effect. For example, as shown in FIG. 2, an initial scene X1 includes a circle, a triangle and a square. In the next scene X2, the square portion in the scene X1 is rewritten into the form of a rectangle. In the next scene Y, the entirety of the scene is changed over to a scene having one large circle. As illustrated in FIG. 3, the sequence of the transmission data for such frame control is as follows: First the data for scene X1 are transmitted, then the data for partially rewriting scene X1 to result in scene X2, and lastly the data for the scene Y. Since the data for rewriting is that involved in partial rewriting of the image of scene X1, the fact that these data follow the image data for the scene X1 is meaningful in the transmission sequence.
With regard to reproducing a recorded image using a playback device or the like, there are cases where a user may require a special reproduction technique, namely so-called reverse reproduction (i.e., playback in reverse order starting from the rear). If the recording device used in a reverse reproduction mode is an optical disk, a head is caused to seek in the reverse direction. If the recording device is a magnetic tape device, the direction of tape movement is -reversed. In other words, in such a special reproduction mode, the sequence of frames to be read from the recording device differs from the sequence at the time of recording. Accordingly, in order to obtain a desired frame in the special reproduction mode, it is required that the address of the storage medium at which the frame is recorded be searched. However, a high-speed search of the recording medium addresses is difficult to realize owing to limitations imposed by the playback rate of the recording medium and by the amount of time required for decoding processing.
Furthermore, even if a high-speed search is possible, in actuality it is difficult in a reverse reproduction mode to reproduce compressively coded data in the reverse direction in the form of a time series, and to reproduce, in a direction entirely the reverse of that of ordinary reproduction, the operation performed at the time of frame changeover, described above. In particular, in the rewriting of a partial frame, the latter unfortunately is reproduced prior to the basic frame (the image composed of one frame), and therefore a normal frame cannot be reproduced.
This problem will be described in further detail with reference to FIGS. 4 and 5.
FIG. 4 shows an example in which an image has been recorded on an optical disk. It is assumed here that basic-frame data and partial-frame data for the 1st through 99th pictures have been recorded on a first track of the optical disk, and that basic-frame data and partial-frame data for 100th through 199th frames have been recorded on a second track of the optical disk. In case of reverse reproduction, a reading head performs a seeking operation from the second track to the first track, but the head reads the frame data in the forward direction on each track. Accordingly, if the playback device does not possess a buffer capacity for 100 scenes when the 100 pictures on the first track are reproduced in the reverse direction, the procedure that must be followed entails reading and reproducing the frame data of the 99th frame, waiting for the disk to make one revolution, and then reading and reproducing the frame of the 98th frame, and so on. Ultimately, the searching of the frame data requires considerable time. The most significant problem arises in a case where the frame data contain the aforementioned partial-frame data. If the first track in FIG. 4 is read in the reverse direction from the 99th frame, then, as shown in FIG. 5, the basic-frame data of the 99th picture is read first, then the partial-frame data for the 96th picture is read as the data of the 98th frame, next the partial-frame data for the 96th frame is read as the data of the 97th frame, and then the basic-frame data for the 96th frame is read as the data of the 96th frame. When it is attempted to reproduce the 98th and 97th frames, rewriting is carried out with regard to the 99th frame. Consequently, the rewritten frame is completely meaningless and presents an abnormal image.
This problem is not limited to a high-definition signal of the aforementioned kind but arises when other types of signals are reproduced as well, such as in the case of television conferences and the like.